Embed pymysql within the collection and use default test container

This change eliminates the need to install the connector on each
controlled node, as `pymysql` version 1.1.1 is now included. As a
result, we can safely assume its availability, thus simplifying the
testing process.

Also, I managed to remove the need for pre-built test containers. We
now use the default test containers from ansible-test.

tests/integration/targets/test_mysql_variables/tasks/mysql_variables.yml

@@ -188,16 +188,14 @@
         output: "{{ oor_result }}"
         var_name: max_connect_errors
         var_value: 1
-      when:
-        - connector_name == 'mysqlclient'
-        - db_engine == 'mysql'  # mysqlclient returns "changed" with MariaDB
 
-    - include_tasks: assert_fail_msg.yml
-      vars:
-        output: "{{ oor_result }}"
-        msg: 'Truncated incorrect'
-      when:
-        - connector_name == 'pymsql'
+    # pymysql apply the invalid value without errors:
+    # msg: "Variable change succeeded prev_value=100"
+    # query: "SET GLOBAL `max_connect_errors` = -1"
+    # - include_tasks: assert_fail_msg.yml
+    #   vars:
+    #     output: "{{ oor_result }}"
+    #     msg: 'Truncated incorrect'
 
     # ============================================================
     # Verify mysql_variable fails when setting an incorrect value (incorrect type)